Theoretical Vacuum Secondary Question

[patrickt]

Originally Posted by Gaz64

Changing from a restrictive air filter proves your own point of the air cleaner was restricting power. The secondaries open sooner as a consequence of a higher venturi vacuum signal across ALL 4 barrels.

Thoughts?

We're in total agreement. My observations on the opening of my secondaries, both before and after the change out to the low restriction Xtreme filter top, were based on "lightly spirited" romps, and how much the maximum opening of the secondaries were during those light romps -- but the car ran as well, or better, than it ever did so I thought nothing of it. This was not WOT, by any means. In fact, back with the small S&H filter, my secondaries would open a bit even on easy, around-town driving. Now with the Xtreme top, they hardly opened at all on the "light romp," but, again, the engine ran beautifully. This weekend I had pushed the zip tie over, to mark the fully closed position, took her for a nice ride, and then looked at the zip tie upon return home and was surprised to see that, instead of having moved the customary half inch or so, it had only moved maybe a third, or maybe even less, from what it has always done. I had had to readjust the mixture when I switched to the new filter top, and I don't know why it didn't occur to me that I would have to eyeball the secondary spring as well... but it didn't. I don't have the quick change unit on the Holley. And I don't remember what spring I put in there years ago, but I do recall it was on the stiff side, say the plain one or maybe the brown one. Changing them out is a bit of a pain and the QF Adjustable Housing looks pretty trick, so I've ordered one. Right now, with the Xtreme filter top and the existing spring, I am pretty much running a two-barrel carb! But what's even more interesting is that there are a bunch of threads on different forums where guys have switched out to the Xtreme and then complained that performance decreased. Nowhere in any of those threads does anyone recommend that you double check your secondaries because you've just decreased venturi vacuum by increasing overall flow. It's actually very cool.